1. Technical Field
The present invention generally relates to a process for separating CO2 from flue gas using amine-functionalized ionic absorbents.
2. Description of the Related Art
One major source of carbon dioxide (CO2) emission is the flue gas that is exhausted as a result of a large industrial combustion process, e.g., refinery heaters and boilers, steam generators, gas turbines, power plants, etc., in large energy consuming industries such as cement, iron and steel and chemical production and oil refining. Accordingly, it is desirable to develop a cost-effective process for separating CO2 from flue gases for the purpose of CO2 capture and sequestration (CCS). This approach is known as “post-combustion” CO2 capture and is applicable to both retrofits and new builds.
Before CO2 can be sequestered from a large industrial source, it must be captured in a relatively pure form. CO2 is routinely separated and captured as a by-product of industrial processes such as synthetic ammonia production, hydrogen (H2) production or limestone calcination. Existing CO2 capture technologies, however, are not cost-effective when considered in the context of sequestering CO2 from large point sources. Most large point sources use air-fired combustors, a process that exhausts CO2 diluted with nitrogen. For efficient carbon sequestration, the CO2 in these exhaust gases must be separated and concentrated.
Currently, the only commercially-proven method of post-combustion capture is through absorption of flue gas with aqueous amine solvents in a packed or trayed absorber column. CO2 is recovered at high purity from the aqueous amine solvents by stream stripping in a packed or trayed regenerator column, and then cooled, dried, and compressed to supercritical pressures for pipeline transport and eventual geologic sequestration. There are many challenges associated with using aqueous amines in “post-combustion” mode.
One such solvent is 30 wt. % monoethanolamine (MEA) in water. Solvent chemistry, corrosion, and viscosity considerations limit the amine strength to about 30 wt. % MEA. At flue-gas CO2 partial pressures (e.g., 0.04 to 0.15 atm), the CO2-rich (“rich”) solvent loading is about 0.42 to 0.45 mol CO2/mol MEA and the CO2-lean (“lean”) solvent loading is about 0.15 to 0.17 mol CO2/mol MEA. The difference in loading (0.25 to 0.3 mol CO2/mol MEA) sets the circulation rate of the amine and influences capital and operating costs.
MEA also has disadvantages in that it has several mechanisms of loss, and a continuous makeup of MEA is required by post-combustion processes. For example, MEA degrades in the presence of oxygen from the flue gas. Thus, to limit the oxidative degradation, corrosion inhibitors may be used. MEA also degrades into heat-stable salts (HSS) from reaction with CO2. To solve this problem, a reclaimer would be added on the regenerator to separate the HSS from the amine solution to provide suitable makeup MEA. Lastly, the volatility of MEA results in the treated flue gas to contain in excess of 500 ppmv MEA when leaving the absorber to the vent. To address this, a wash section is added at the top of the absorber and makeup water is added to scrub the MEA from the treated flue gas. The mixture is then sent down the column along with the remaining lean solvent to absorb CO2 from the incoming flue gas. Water washing can cut the MEA emissions to about 3 ppmv.
MEA may also degrade over time thermally, thereby limiting the temperature of operation in the absorber and regenerator. With a cooled flue gas inlet temperature of about 56° C., the absorber column may operate at a bottoms temperature of 54° C. and a pressure of 1.1 bar while the regenerator may operate at a bottoms temperature of 121° C. (2 bar saturated steam) at 1.9 bar. For 30 wt. % MEA, the amine reboiler steam temperature is kept at less than 150° C. (4.7 bar saturated steam) to limit thermal degradation.
MEA also degrades in the presence of high levels of NOx and SOx which are common in facilities that burn coal and fuel oil. However, if CO2 removal from a high NOx and SOx containing flue gas is desired, separate process facilities such as SCR (Selective Catalytic Reduction) and FGD (Flue Gas Desulfurization) are needed for removal of NOx and SOx, respectively. In order to have a superior, post-combustion CO2 removal technology that is better than those known in the art (30 wt. % MEA and similar aqueous amines), an improved CO2-removal solvent and process using the solvent is required.
Ionic Liquids (IL) are a class of compounds that are made up entirely of ions and are liquid at or below process temperatures. ILs are known in the art to have vanishingly low or negligible vapor pressure, and are often studied as candidates for environmentally-benign solvents, catalysts, and gas and liquid-phase separation agents. Many ILs known in the art behave as physical solvents, meaning that the loading capacity of CO2 is linear with the equilibrium partial pressure of CO2. These ILs are unsuitable for flue-gas applications since the partial pressure of CO2 is from about 0.04 to about 0.15 atm and are better suited for highly-concentrated CO2 applications such as syngas. Other ILs appear more active for CO2 at lower partial pressures and have non-linear loading curves, for example, bmim acetate as disclosed in U.S. Pat. No. 7,527,775. The anions of this type of ILs are able to interact favorably with CO2 and thereby have higher loadings at low partial pressure.
More recently, researchers have developed task-specific IL (TSIL) and other forms of functionalized ILs that have chemical functional group designed to interact even more strongly and/or specifically with CO2. Although these amine-functionalized ILs are known to have a high capacity for CO2, they are also known to be far more viscous compared to non-functionalized ILs. However, a high viscosity is undesirable on a commercial scale, because it means that pumping costs are higher, mass transfer kinetics between gas and liquid are lower (resulting in taller columns and more packing material), and the efficiency of heat transfer is reduced (needed for eventual regeneration).
Accordingly, there is a continued need for improved systems and processes for removing CO2 from flue gases that can be carried out in a simple, cost-effective manner.